1. Field of the Invention
The present invention relates to an optical pickup for a digital versatile disk (DVD) which is compatible with a class of compact disks, and more particularly, to an optical pickup capable of recording and reproducing a signal with respect to a digital versatile disk (DVD) and a class of compact disks, by moving a position of a laser unit which emits a light beam having a relatively longer wavelength than that used in conjunction with the DVD and detects the light beam having the relatively long wavelength reflected from a compact disk, and limiting a size of a photodetector of the laser unit which detects the reflected light beam.
2. Description of the Related Art
So far, a high-density optical disk system enlarges a numerical aperture of an objective lens in order to increase a recording density, and uses a short wavelength light source of 635 nm or 650 nm. The system has been developed to also reproduce information from a compact disk (CD) having a thickness different from that of a digital versatile disk (DVD) as well as recording information to and reproducing information from a DVD, by using a short wavelength light source. However, to be compatible with a compact disk-recordable (CD-R), which is a recent type of a CD, laser light having a wavelength of 780 nm should be used. This is due to the recording characteristic of the CD-R as a recording medium. As a result, using light beam wavelengths of 780 nm and 635 nm in a single optical pickup becomes very important for compatibility of the DVD and the CD-R. A conventional optical pickup which is compatible with the DVD and the CD-R will be described below with reference to FIG. 1.
FIG. 1 shows an optical pickup using two laser light sources and a single objective lens. The optical pickup shown in FIG. 1 uses laser light having a wavelength of 635 nm when reproducing information from a DVD, and uses laser light having a wavelength of 780 nm when recording information from and reproducing information to a CD-R.
A light beam having a wavelength of 635 nm emitted from a laser light source 11 is incident to a collimating lens 12. The light beam is depicted as a solid line. The collimating lens 12 collimates the incident light beam emitted from the laser light source 11 into a parallel light beam. The parallel light beam passing through the collimating lens 12 is reflected by a beam splitter 13, and then goes to an interference filter prism 14.
Meanwhile, a light beam having a wavelength of 780 nm emitted from a laser light source 21 passes through a collimating lens 22, a beam splitter 23 and a converging lens 24 in sequence, and then proceeds to the interference filter prism 14, which is depicted as a dotted line.
The interference filter prism 14 totally transmits the light beam having the wavelength of 635 nm reflected by the beam splitter 13, and totally reflects the light beam of the 780 nm wavelength converged by the converging lens 24. As a result, the light beam emitted from the laser light source 11 is incident to a wave plate 15 in the form of a parallel beam. The light beam emitted from the laser light source 21 is incident to the wave plate 15 in the form of a divergent beam. The light beams transmitted via the wave plate 15 pass through a variable aperture 16, and are then incident to the objective lens 17.
The objective lens 17 is designed to focus a light beam of the 635 nm wavelength emitted from the laser light source 11 on an information recording surface of a DVD 18 having a thickness of 0.6 mm.
Therefore, the light beam reflected from the information recording surface of the DVD 18 contains information recorded on the information recording surface thereof. The reflected light beam is transmitted through the objective lens 17, the variable aperture 16, the wave plate 15, the interference filter prism 14, and the beam splitter 13, and then is incident to a photodetector 19 for detecting optical information.
Also, the objective lens 17 focuses a divergent light beam having the wavelength of 780 nm emitted from the laser light source 21 on an information recording surface of a CD-R 25 having a thickness of 1.2 .mu.m. An optical system having such a structure which can converge a divergent light beam using the objective lens 17, is called a "finite optical system".
Spherical aberration is generated due to a difference in the thicknesses between the DVD 18 and the CD-R 25. In more detail, the spherical aberration is due to the fact that the distance between the information recording surface of the CD-R 25 and the objective lens 17 is farther than that between the information recording surface of the DVD 18 and the objective lens 17, along an optical axis.
By using the variable aperture 16 to be described later with reference to FIG. 2, the light beam of the 780 nm wavelength forms an optimized beam spot on the information recording surface of the CD-R 25. The light beam of the 780 nm wavelength reflected from the CD-R 25 is transmitted through the objective lens 17, variable aperture 16, the wave plate 15, reflected by the interference filter prism 14, transmitted through the converging lens 24 reflected by the beam splitter 23, and then is detected in the photodetector 26.
The variable aperture 16 of FIG. 1 has a thin-film type structure as shown in FIG. 2 which can selectively transmit the light beams incident to the region whose numerical aperture (NA) is less than or equal to 0.6 which coincides with the diameter of the objective lens 17. That is, the variable aperture 16 is partitioned into two regions. A first region is a region which totally transmits the light beams of the 635 nm wavelength and the 780 nm wavelength and whose numerical aperture (NA) is less than or equal to 0.45. A second region surrounding the first region is a region which includes a dielectric thin film, totally transmits the light beam of the 635 nm wavelength and totally reflects the light beam of the 780 nm wavelength, and whose numerical aperture (NA) is greater than 0.45. Also, the first region comprises a quartz (SiO.sub.2) thin film in order to remove any optical aberration generated by the dielectric thin film coated second region.
The 780 nm wavelength light passing through the first region having the 0.45 NA or below in the variable aperture 16 forms a beam spot appropriate to the CD-R 25 on the information recording surface thereof. Thus, the optical pickup of FIG. 1 forms an optimized optical spot and is compatible with the DVD 18 and the CD-R 25 when an optical recording medium is changed from the DVD 18 and the CD-R 25.
The optical pickup of FIG. 1 as described above should form a "finite optical system" with respect to the light beam of the 780 nm wavelength in order to remove spherical aberration generated when compatibly changing a DVD and a CD-R. However, such a fabricating process of the optical system does not only become complicated but also assembly of the various optical components is difficult. Also, an optical path difference between the first region having the 0.45 NA or below and the second region having the 0.45 NA or above, is generated due to the dielectric thin film formed at the second region having the 0.45 NA or above in the variable aperture 16. Therefore, the first region should include a special optical thin film, i.e., a quartz (SiO.sub.2) thin film, to remove this difference. For this reason, the first region includes the quartz (SiO.sub.2) thin film and the second region includes the dielectric thin film. However, a manufacturing process is complicated and adjustment of the thicknesses of the quartz (SiO.sub.2) thin film and the dielectric thin film should be performed precisely in units of ".mu.m." Thus, it has been difficult in mass-producing the above-described optical pickup.
In order to solve the above problems, in the case of forming an optical pickup (not shown) including a laser unit in which the laser light source 21 for emitting the light beam of the 780 nm wavelength and the photodetector 26 are incorporated in a single module, without using the variable aperture 16 and the converging lens 24, spherical aberration is not generated with respect to the CD-R 25. And, the optical pickup forms a finite optical system maintaining the 0.55 NA or above in a numerical aperture of the objective lens, and thus a beam spot having a size of 1.2-1.3 .mu.m is formed on the information recording surface of the CD-R 25. However, a most appropriate size of the beam spot when reproducing information from general types of compact disks is 1.4.about.1.6 .mu.m. To increase the size of a beam spot converged into the CD-R, the laser unit is moved at a position where no spherical aberration is generated so that a total conjugate length (TCL) is lengthened. The TCL is a distance corresponding to an optical path from an information recording surface of a disk to a laser light source. A beam spot having the size of 1.4 .mu.m is formed on the information recording surface of the CD-R according to movement of the laser unit, but spherical aberration is generated and a side lobe is increased. The light beam generating such a side lobe is a light beam passing through a far region of the objective lens having a relatively larger radius than that of a near axis region of the objective lens, and has a negative effect on a reproduction signal.